Red-emitting phosphors that are excited by light corresponding to near-UV to blue light-emitting diodes (LEDs) are needed to improve the color rendering properties of white LEDs or, when white LEDs are used as the backlights in liquid crystal displays, to enhance color reproducibility, and are being studied for this purpose. In particular, JP-A 2009-528429 (Patent Document 1) mentions that complex fluoride phosphors obtained by adding manganese to a complex fluoride of the formula A2MF6 (wherein A is, for example, sodium, potassium or rubidium; and M is, for example, silicon, germanium or titanium) are useful.
Patent Document 1 discloses one method for producing such phosphors wherein a hydrofluoric acid solution in which all of the constituent elements have been dissolved or dispersed is concentrated by evaporation, inducing precipitation. Another production process, disclosed in U.S. Pat. No. 3,576,756 (Patent Document 2), is a method wherein hydrofluoric acid solutions in which the respective constituent elements have been separately dissolved are mixed together, following which the water-soluble organic solvent acetone is added, thereby lowering solubility and inducing precipitation. In addition, JP No. 4582259 (Patent Document 3) discloses a method wherein the element M and the element A in the above formula are each dissolved in separate hydrofluoric acid-containing solutions, manganese is added to either one of these solutions, and the solutions are mixed together, thereby inducing precipitation of the phosphor.
In processes for producing the above known manganese-doped complex fluoride phosphors of the formula A2MF6 (wherein A is, for example, Na, K or Rb; and M is, for example, Si, Ge or Ti), including in the foregoing literature, the manganese is added in the form of a compound (hexafluoromanganate(IV)) of the formula A2MnF6 (wherein A is, for example, Na, K or Rb), especially as K2MnF6, in the phosphor-forming operation. The production of K2MnF6 by a method discovered by H. Bode et al. (H. Bode, H. Jenssen, F. Bandte: Angew. Chem. 65, p. 304 (1953); Non-Patent Document 1), that is, by a method in which an aqueous solution of hydrogen peroxide (H2O2) is added little by little to a solution obtained by dissolving potassium fluoride (KF) and potassium permanganate (KMnO4) in hydrofluoric acid, has also been reported
Another known method for producing A2MnF6 compounds is the method, described by R. Hoppe et al. (R. Hoppe, W. Liebe, W. Daehne: Z. Anorg. Allg. Chem. 307, p. 276 (1961); Non-Patent Document 2), of heating a mixture of an alkali metal chloride and anhydrous manganese(II) chloride to about 400° C. in a stream of fluorine gas. In addition, B. Cox et al. (B. Cox, A. G. Sharpe: J. Chem. Soc., p. 1798 (1954); Non-Patent Document 3) state that production can be carried out by inserting a platinum electrode in a manganese(II) fluoride-containing hydrofluoric acid solution and carrying out electrolysis. The production is also cited and discussed in Shin Jikken Kagaku K●za 8 [New experimental chemistry series 8] (edited by The Chemical Society of Japan): “Inorganic Compound Synthesis III,” p. 1166 (Maruzen, Inc., 1977); Non-Patent Document 4).
However, the Bode method (Non-Patent Document 1) requires precise control, such as adding hydrogen peroxide little by little and keeping the temperature low, so that the reaction does not proceed to excess, and so carrying this method out industrially is technically demanding. Moreover, when control is inadequate, the quality of the resulting product also is inadequate. As a result, the characteristics of phosphors obtained using this method may be somewhat unsatisfactory. As for the Hoppe method (Non-Patent Document 2), because it uses fluorine gas, which is highly toxic, strongly corrosive and thus dangerous, it would likely require special equipment and facilities to carry out industrially. From this standpoint, the method using electrolysis that is based on the research by Cox et al. (Non-Patent Documents 3, 4) appears to be suitable for industrial use. The inventors, too, have disclosed, in JP-A 2012-224536 (Patent Document 4), the production of complex fluoride phosphors using K2MnF6 produced by this electrolytic method. However, at the time, they were still investigating the electrolytic reaction conditions that affect the quality of the K2MnF6 and of the phosphors produced using this.